The present invention relates to a process for the manufacture of potassium sulphate from solutions containing magnesium chloride, such as solutions of carnallite ore and, in particular, the equilibrium mother liquors of a unit for the treatment of carnallite.
Carnallite is a salt having the following composition: KCl.MgCl.sub.2.6H.sub.2 O. It is found in solid form in a certain number of deposits, and it is present, in the form of impurities, in larger or smaller amounts, in a certain number of other potassium deposits. Otherwise, it can be crystallized from natural brines.
Whatever its origin, carnallite is brought into contact, for treatment, with a decomposition mother liquor having a content of MgCl.sub.2, KCl and NaCl such that potassium chloride and sodium chloride (synthetic sylvinite) are precipitated in an equilibrium mother liquor. This equilibrium mother liquor is saturated with potassium chloride and sodium chloride, and it is more or less saturated with magnesium chloride.
This equilibrium mother liquor is frequently discarded, but, in that case, the potassium which it contains is lost and the potassium yield of the unit is poor. Attempts have been made to concentrate this equilibrium mother liquor in order to recover an additional amount of carnallite which is recycled. In practice, however, this evaporation is economical only to the extent that it can be carried out by natural evaporation, for example, in solar evaporation pools.